1. Field of the Invention
The present invention relates to a cooling device for use in an electronic apparatus such as a liquid crystal projector, and more particularly to a cooling device for cooling the surface of a member which gives off heat.
2. Description of the Related Art
Cooling devices comprising a fan for forced air cooling are widely used as cooling means for electronic apparatus because they are inexpensive and simple in structure. Liquid crystal projectors also employ a cooling device which comprises a plurality of fans for forced air cooling of optical components which give off heat in the liquid crystal projectors.
In the liquid crystal projectors, a light beam from a light source is applied to a polarizing beam splitter. The light beam polarized by the polarizing beam splitter is separated into red, green, and blue light beams corresponding to the three primary colors. The red light beam is applied to a red liquid crystal panel, the green light beam to a green liquid crystal panel, and the blue light beam to a blue liquid crystal panel. The liquid crystal panels optically modulate the respective applied light beams with a video signal. The optically modulated light beams are combined by a color combining prism into a single light beam, which is projected onto a screen by a projecting lens.
Each of the liquid crystal panels comprises a matrix of liquid crystal cells and a light shielding area called a “black matrix” surrounding each of the liquid crystal cells. When the black matrix absorbs light, the liquid crystal panel produces heat.
If the liquid crystal panels comprise liquid crystal panels that operate in a TN (Twisted Nematic) mode, then polarizer plates are disposed respectively on the entrance and exit sides of each of the liquid crystal panels. The polarizer plates pass only certain polarized light, e.g., S-polarized light, and block other light. When the polarizer plates block light, they convert the light into heat. Therefore, the polarizer plates also produce heat.
The liquid crystal panels and the polarizer plates are often made of an organic material. Therefore, if the liquid crystal panels and the polarizer plates are kept at high temperatures for a long period of time, then alignment layers of the liquid crystal panels may be damaged and polarization selectivity may be lowered. Therefore, liquid crystal projectors incorporate cooling devices for cooling components that give off heat, such as liquid crystal panels and polarizer plates.
A cooling device for use in liquid crystal projectors will be described below. Liquid crystal panels and polarizer plates disposed on the entrance and exit sides of the liquid crystal panels will hereinafter be referred to as “a liquid crystal unit”.
FIGS. 1A through 1C are views showing a liquid crystal projector according to the related art. FIG. 1A is a perspective view of the liquid crystal projector, FIG. 1B is a perspective view showing internal structural details of the liquid crystal projector, and FIG. 1C is a view showing the layout of components of the liquid crystal projector.
As shown in FIGS. 1A through 1C, liquid crystal projector 1 has a casing which houses therein liquid crystal unit assembly 2, cooling fan 3, air cooling duct 4, light source 5, power supply unit 6, lamp cooling fan 7, exhaust fan 8, and projecting lens 9.
Liquid crystal unit assembly 2 comprises R, G, B liquid crystal units. Each of the liquid crystal units comprises a liquid crystal panel and polarizer plates disposed on the entrance and exit sides of the liquid crystal panel. A light beam from light source 5 is separated into red, green, and blue light beams by a plurality of dichroic mirrors. The separated red, green, and blue light beams are supplied to the respective liquid crystal units of liquid crystal unit assembly 2. The red, green, and blue light beams are spatially modulated by the liquid crystal units into image light beams, which are combined by a color combining prism. The combined image light beam from the color combining prism is projected onto a screen by projecting lens 9.
Lamp cooling fan 7 serves as a means for cooling light source 5. An air stream produced by lamp cooling fan 7 flows through a lamp cooling duct to light source 5. Cooling fan 3 and air cooling duct 4 serve as a means for cooling liquid crystal unit assembly 2. An air stream produced by air cooling fan 3 flows through air cooling duct 4 to the liquid crystal units of liquid crystal unit assembly 2. Exhaust fan 8 discharges air in the casing out of the casing.
FIGS. 2A and 2B are views showing specific structural details of a cooling device for cooling liquid crystal units. FIG. 2A is an exploded perspective view of the cooling device, and FIG. 2B is a cross-sectional view illustrative of a cooling action of the cooling device.
As shown in FIGS. 2A and 2B, liquid crystal unit assembly 2 comprises three liquid crystal units. Each of the liquid crystal units comprises liquid crystal panel 11 and polarizer plates 10, 12 disposed respectively on the entrance and exit sides of liquid crystal panel 11.
Cooling device 13 comprises air cooling fan 3 and air cooling duct 4. Air cooling duct 4 has a portion positioned below liquid crystal unit assembly 2 and having three outlet ports 15 for ejecting air streams toward the respective liquid crystal units. The air streams flow through air cooling duct 4 and are directed from respective outlet ports 15 toward the respective liquid crystal units. The air streams from outlet ports 15 pass upwardly through spaces (gaps) between polarizer plates 10 and liquid crystal panels 11 and between liquid crystal panels 11 and polarizer plates 12. As the air streams flowing out of outlet ports 15 pass through the gaps between liquid crystal panels 11 and polarizer plates 10, 12, they cool liquid crystal panels 11 and polarizer plates 10, 12.
JP-A No. 11-295814 discloses another cooling device for cooling a liquid crystal unit. FIG. 3 is a view showing structural details of the disclosed cooling device.
As shown in FIG. 3, air streams from cooling fan 3 flow between polarizer plate 10 and liquid crystal panel 11 and between liquid crystal panel 11 and color combining prism 16. Air deflecting plate 17 for changing the direction of the air stream from cooling fan 3 is mounted on a portion of a member which holds color combining prism 16 near cooling fan 3. The air stream from cooling fan 3 has its direction changed by air deflecting plate 17, and is directed toward the surface of liquid crystal panel 11. The direction of the air stream is thus changed to improve the cooling efficiency of liquid crystal panel 13.
JP-A No. 2001-318361 discloses still another cooling device for cooling a liquid crystal unit. FIG. 4 is a view showing structural details of the disclosed cooling device.
As shown in FIG. 4, liquid crystal panel 11 is held by holding frame 18 having two protrusions 19 along the opposite edges thereof. Protrusions 19 are in the form of plate-like members for limiting an air stream supplied from duct outlet port 20 to flow in one direction, thereby keeping a flow rate (air rate) of the air stream flowing along the surface of liquid crystal panel 11.
JP-A No. 2000-124649 discloses yet another cooling device for cooling a liquid crystal unit. FIGS. 5A and 5B are views showing structural details of the disclosed cooling device. FIG. 5A is a plan view, and FIG. 5B is a sectional side elevational view.
As shown in FIGS. 5A and 5B, air guide 21 having a U-shaped cross section is mounted on and extends between color combining prism 16 and polarizer plate 10 that are disposed in confronting relation to each other with liquid crystal panel 11 interposed therebetween. An air stream from cooling fan 3 passes between color combining prism 16 and liquid crystal panel 11, and thereafter is caused to flow back by air guide 21. The air stream caused to flow back by air guide 21 passes between liquid crystal panel 11 and polarizer plate 10. This structure is effective to prevent temperature irregularities on the surface of liquid crystal panel 11.
JP-A No. 2000-124649 also discloses a modification of the above cooling device. FIGS. 6A and 6B are views showing such a modification. FIG. 6A is a plan view, and FIG. 6B is a sectional side elevational view. According to the modification, lower cooling fan 22 is disposed below liquid crystal panel 11, and upper cooling fan 23 is disposed above liquid crystal panel 11. An air stream from lower cooling fan 22 passes upwardly between color combining prism 16 and liquid crystal panel 11. An air stream from upper cooling fan 23 passes downwardly between liquid crystal panel 11 and polarizer plate 10. The modification is also effective to prevent temperature irregularities on the surface of liquid crystal panel 11.
Generally, attempts to improve the heat transfer coefficient for promoting the heat transfer in forced air cooling for a heated flat plate include two approaches, “thinned layer method” and “replacement method”.
The former “thinned layer method” is a method of promoting the heat transfer from a heated body to a coolant (air) by thinning a thermal boundary layer (thinned layer) on the surface of the heated body. Since the thickness of the thermal boundary layer is inversely proportional to the square root of the velocity in the direction of a main flow (the flow velocity of a flow along the surface of a flat plate), the flow velocity may be increased for lowering the temperature of the heated body.
However, if the flow velocity from a fan is increased for the purpose of improving the cooling capability, then the operating noise of the fan becomes worse and the volume of the fan increases. Furthermore, inasmuch as the heat transfer coefficient is proportional to the square root of the flow velocity (=thickness of the thermal boundary layer is inversely proportional to the square root of the flow velocity) (laminar flow), the thinned layer method is problematic in that if the temperature is lowered to a certain level, then it will not be significantly lowered further no matter how much the flow velocity is increased (air cooling limitation). The cooling device shown in FIGS. 2A and 2B and the cooling devices disclosed in JP-A No. 2001-318361 and JP-A No. 2000-124649 are classified as cooling devices according to the method of cooling a heated flat plate based on the “thinned layer” method. These cooling devices are facing the above problem as liquid crystal projectors are required to be smaller in size, higher in luminance, and longer in product-life cycle.
The latter “replacement method” is a method of promoting the heat transfer by creating a turbulent flow of air to accelerate the generation/elimination of unsteady vortexes for thereby forcibly exchanging a fluid near the surface of the heated body (high temperature) and a fluid spaced a little from the surface of the heated body (low temperature).
One typical example of the “replacement method” is impinging jet cooling. The impinging jet cooling refers to a cooling method for causing a jet (a coolant such as water or air) from a nozzle to impinge perpendicularly upon a heated flat plate to radiate heat therefrom.
According to the impinging jet cooling, the heated surface is effectively cooled by the following three processes:
1) the breakage (peeling) of the thermal boundary layer on the surface of the heated body due to the impingement of the jet;
2) the fluid exchange (temperature replacement) due to swirling vortexes generated on the impinging surface; and
3) the slippage on the wall surface of the jet due to the Coanda effect.
The Coanda effect refers to the property of a fluid such that when an object is placed in a fluid flow, the pressure between the fluid and the solid wall surface of the object drops to attract the fluid flow to the wall surface, causing the fluid to flow along the solid wall surface of the object.
If the impinging jet cooling is applied to a process of cooling the liquid crystal units of a liquid crystal projector, then the position of the nozzle for producing the jet is of importance. Specifically, since the liquid crystal panels and the polarizer plates give off heat as they absorb light passing therethrough, the surface which gives off heat and the surface through which light passes essentially coincide with each other. Therefore, it is important to generate an air flow (impinging jet) of air perpendicularly to the heated surface so as not to block the transmission of the light in the small gaps between the liquid crystal panels and the polarizer plates.
According to the cooling device disclosed in JP-A No. 11-295814, the air deflecting plate at the duct outlet controls the amount and direction of air applied to the liquid crystal panel to improve the cooling capability. The disclosed cooling device is classified into something between the “thinned layer method” and the “replacement method”. Since the amount of air applied to the polarizer plate which is positioned opposite to the liquid crystal panel is reduced, the cooling capability for the polarizer plate is lowered. For tilting the air flow between the liquid crystal panel and the polarizer plate with the air deflecting plate at the duct outlet, the liquid crystal panel and the polarizer plate need to be spaced sufficiently apart from each other. Unless they are spaced sufficiently apart from each other, the air flow therebetween cannot effectively be tilted, and the flow path is closed instead, resulting in a reduction in the cooling capability. Furthermore, even if the air is delivered obliquely to the liquid crystal panel, its cooling capability is far from the cooling capability which would be achieved by the impinging jet.